Problem: The scale on a map is 6cm : 10km. If two cities are 54cm apart on the map, what is the actual distance in km between the two cities?
Explanation: The scale means that every 6cm on the map corresponds to an actual distance of 10km. A distance of 54cm on the map is the same as 9 $\cdot$ 6cm. The actual distance between the two cities is 9 $\cdot$ 10km, or 90km.